1. Technical Field
The present invention relates generally to a multilayer secondary battery with a plurality of flat-form electrodes stacked together via a separator, and more particularly to a multilayer secondary battery wherein a positive electrode and a negative electrode differing in electrode area are stacked together via a separator and a method of making the same.
The invention is also concerned with a lithium ion secondary battery with a positive electrode smaller in size than a negative electrode and a method of making the same.
2. Related Art
Lithium ion batteries known until now are broken down into a wound-up type battery comprising an outer container containing a wound-up battery element with a positive electrode and a negative electrode, each in belt form, stacked together via a separator, and a multilayer secondary battery wherein an outer container contains a battery element comprising a positive electrode and a negative electrode, each in flat sheet form, stacked together via a separator.
FIG. 6 is a sectional view of a battery element in a prior art multilayer secondary battery, as sectioned in a vertical direction to an electrode plane.
A multilayer body of a battery element shown generally at 5 that forms part of a multilayer secondary battery comprises a positive electrode 2 and a negative electrode 3, each in flat sheet form, stacked together via a separator 4. The flat sheet-form positive electrode 2 has a positive electrode active substance layer 2B formed on a positive electrode collector 2A, a part of which extends outwardly from its portion facing the opposite electrode in the form of a positive electrode lead terminal 7. Likewise, the negative electrode 3 has a negative electrode active substance layer 3B on a negative electrode collector 3A, a part of which extends outwardly from its portion facing the opposite electrode in the form of a negative electrode lead terminal 8.
When, in manufacturing an electrode element by stacking the positive electrode and the negative electrode together via the separator, the multilayer electrode element is contained in an outer container such as a metal can or sealed up with an outer casing member having flexibility, it is found that there is a misalignment because the flat sheet-form positive electrode 2, the flat sheet-form negative electrode 3 and the separator 4 are each of independent structure, offering problems that internal short circuits occur upon partial direct contact of the positive electrode 2 with the negative electrode 3, or a misalignment between the positive electrode 2 and the negative electrode 3 fails to give any desired properties in terms of the charge/discharge capacity, etc. of the battery.
As the size of the separator is increased to get around internal short circuits even upon some electrode misalignment, the size of the outer container grows large, resulting in problems that battery products grow bulky, and the volume capacity density of the battery decreases.
For a secondary battery, it is required that the positive electrode and the negative electrode be located in such a way as to conduct uniform charge/discharge currents through any electrode plane site. Especially because currents are likely to concentrate at the corners of electrode ends, it is demanded to avert such concentration of currents at the electrode ends.
Especially when it comes to a lithium ion battery, concentration of currents at the corners of a negative electrode end during overcharging or the like causes dendrite to be formed, breaking through the separator and bringing about internal short circuits between the negative electrode and the positive electrode, or posing other problems.
For the lithium ion battery, therefore, the area of the negative electrode is designed to be larger than that of the opposing positive electrode for the purpose of preventing concentration of currents at the end of the negative electrode during charging.
FIGS. 7A, 7B and 7C are illustrative in plan of the sizes of the positive electrode, the negative electrode and the separator, respectively.
Among the length 81 and width 82 of a positive electrode 52 depicted in FIG. 7A, the length 83 and width 84 of a negative electrode depicted in FIG. 7B and the length 85 and width 86 of a separator depicted in FIG. 7C, there are relations given bypositive electrode length 81<negative electrode length 83≦separator length 85positive electrode width 82<negative electrode width 84≦separator width 86
Accordingly, difficulty would be encountered in stacking the components without any lateral misalignment even on the basis of any component.
To prevent any electrode misalignment during stacking with this in mind, JP-A-2002-252023 proposes a multilayer secondary battery wherein an electrode having a smaller area is covered on both sides with a separator and the outer peripheral size of the separator is the same as the size of an electrode having a larger area.
However, when the periphery of the separator is thermally fused while the electrode of smaller area is covered on both surfaces with the separator, it is not easy to do this in such a way as to have the positive electrode fixed inside with no wasteful space in the separator. It is also difficult to make the thermally fused width small enough to have no influence on battery performance.